This invention relates to knife and blade sharpeners of the type that use disk type sharpening members.
Household and commercial knife sharpeners incorporate high-speed cylindrical stones rotating at surface velocities of 2,000 feet per minute (U.S. Pat. No. 2,775,075.) These sharpeners generally leave a large burr (i.e. a curled-over edge of metal on the last unsharpened facet of the blade edge) and tend to overheat the edge. The presence of a sizeable burr is undesirable since it is ragged, non-uniform and creates a weak edge.
U.S. Pat. No. 4,627,194 describes sharpeners that overcame the problem of a large burr in a particular way by adding additional facets (typically 3) where each succeeding facet has a slightly larger included angle. Furthermore, each succeeding facet is formed with a finer grit until a very fine grit is used on the last (third) facet. Thus burrs never become large or excessive. This action, while producing a well formed strong edge takes more time than is desirable in commercial environments.
Prior art sharpeners that create large burrs require post treatment as with a steel rod or a cloth buffing wheel to align the burr. Sharpening steels and cloth wheels are time consuming and generally leave a weak edge that will need frequent resharpening.
Many of the prior art sharpeners of the household type provide poor control of the blade position and angle of the knife edge facet relative to the sharpening disk. U.S. Pat. No. 4,627,194 describes a magnetic hold down mechanism, but it is possible for the user to forcibly override the magnetic field. Other prior art sharpeners, used commercially, provide complex and unreliable clamping mechanisms to insure proper knife position and angle of the cutlery edge facet. Users find these cumbersome and difficult to use.
Another disadvantage of many prior art sharpeners is that they provide no control of the force of the grinding surface on the knife facet. Thus the speed of sharpening will vary depending on blade thickness. Furthermore, this lack of control can result in the user applying too much force, thus overheating the knife edge locally, degrading the edge temper or creating a nonuniform cutting edge.